Vehicle drive-trains known from practice, in each case built with a drive engine, a transmission device and a drive output, often have transmissions designed as hydrostatically power-branched powershift transmissions. During overdrive operation of the vehicle drive-train, it is known that during a deceleration process of a vehicle built with the vehicle drive-train or while driving downhill, torque resulting from the moving vehicle mass and applied at the drive output is transmitted from the drive output, via the transmission, in the direction toward the drive engine. The applied thrust torque is absorbed by a hydraulic motor of the transmission device and is then supported by a hydraulic pump device in the area of the drive engine, the latter usually being in the form of an internal combustion engine. If the supporting torque available in the area of the drive engine during overdrive operation of the vehicle drive-train is too low, in some circumstances this results in an undesirably large increase of the rotational speed of the drive engine.
In order to avoid such rotational speed increases in the area of the drive engine, a driver of the vehicle built with such a vehicle drive-train has to actuate the service brakes. In vehicle systems that assist the driver, when the drive engine runs at undesirably high rotational speeds the service brakes are automatically actuated in order to limit the rotational speed of the drive engine.
However, particularly during longer downhill driving stretches, this is not desired since particularly when the service brakes are actuated automatically, overloading can occur in the area of the service brakes.
A method and a device for controlling a motor vehicle drive-train are known from the document EP 1 097 318 B1. The motor vehicle drive-train is built with a hydrostatically-mechanically power-branched transmission whose transmission ratio is continuously variable. If the driver calls for a driving direction reversal while moving at above a predetermined speed limit, in the method the driving speed is reduced by continuously regulated and controlled downshifting of the transmission ratio in the area of the transmission. When the speed reaches or falls below the speed limit, the previously open clutch for the new driving direction is brought by pressure modulation to a frictional slipping condition and, overlapping this frictional slipping operation in time, the closed and slip-free clutch for the previous driving direction is also brought to a frictional slipping condition by reducing its closing pressure, so that the drive torque is transferred without interruption from the old clutch to the new clutch. The friction work of the clutch for the old driving direction can be used to produce a braking torque additional to the braking power of the engine.
Furthermore, it is proposed to bring the clutch of the previous driving direction to frictional slipping operation, whereas the clutch for the new driving direction is only brought to slipping operation when the speed limit has been reached, in order to avoid excessive thermal loading. During this the frictional slipping condition of the old driving direction clutch should be maintained until the driving speed of the vehicle has decreased sufficiently to enable further deceleration to be effected by the clutch for the new driving direction.
Even this procedure is not suitable, to the desired extent, for avoiding an unacceptably high rotational speed in the area of the drive engine during overdrive operation of a vehicle while at the same time the thrust torque is high.